The present invention relates to a method and apparatus for continuous generation of a laser beam comprising ultrashort pulses having a pulse width of 1 fs or shorter on the basis of stimulated Raman scattering and four-wave Raman mixing.
The inventors of the present invention have proposed a method and apparatus for generating ultrashort optical pulses by means of four-wave Raman mixing, as disclosed in (1) Japanese Patent Application Laid-Open (kokai) No. 6-21550 and (2) Japanese Patent Application Laid-Open (kokai) No. 10-133241.
However, such a method of generating ultrashort optical pulses through four-wave Raman mixing requires employment of optical pulses having a high peak as excitation light.
Moreover, in the method, generated Raman light is subjected to pulse compression by use of a Fourier-transform-limited pulse having pulse width and linewidth narrowed to the theoretical limit. Therefore, the problem of easy occurrence of non-linear optical effects such as self-focusing, self-phase modulation, and harmonic generation remains unsolved.
An object of the present invention is to solve the above-described problem and provide a method and apparatus for generating ultrashort optical pulses through use of a Raman resonator, which can generate preferentially rotational Raman lines which resonate within the resonator, which can lower the threshold for generation of rotational Raman lines to thereby increase efficiency, and which enables generation of high-order rotational Raman lines to thereby obtain ultrashort laser pulses.
To achieve the above object, the present invention provides the following:
[1] A method for generating ultrashort optical pulses by use of a Raman resonator, in which a continuous-wave laser of a predetermined frequency, serving as excitation light, is focused into a resonator having a specific resonator length determined by the frequency and is phase-locked in order to generate preferentially rotational Raman lines which resonate within the resonator.
[2] A method for generating ultrashort optical pulses by use of a Raman resonator, in which two laser beams of different colors, which have a predetermined frequency separation therebetween and serve as excitation light, are focused into a resonator having a predetermined resonator length, such that rotational Raman lines are generated within the resonator through four-wave Raman mixing, while vibrating Raman lines are suppressed.
[3] The method for generating ultrashort optical pulses by use of a Raman resonator described in [2] above, characterized in that the predetermined frequency separation is 587 cmxe2x88x921.
[4] An apparatus for generating ultrashort optical pulses by use of a Raman resonator, the apparatus comprising a first laser; a second laser; a first wavemeter for confirming the emitting wavelength of the first laser; a second wavemeter for confirming the emitting wavelength of the second laser; and a Raman cell, two laser beams from the first and second lasers being aligned by a beam splitter to become coaxial and focused by a lens into the Raman cell.
[5] The apparatus for generating ultrashort optical pulses by use of a Raman resonator described in [4] above, characterized in that each of the lasers is a semiconductor laser.
[6] The apparatus for generating ultrashort optical pulses by use of a Raman resonator described in [4] above, characterized in that each of the lasers is a titanium:sapphire laser.